The reduction in the radar signature of an aircraft has obvious military advantages. This can generally be accomplished in two ways. The first is active electronic countermeasures which jam, disrupt, or otherwise confuse the reflected signal. The second method is to use passive techniques which basically divide into two categories: (1) the shape of the aircraft and (2) the use of radar absorption material coatings, and those coatings in combination with substrate structures which also absorb the incoming radar signal and reflect back only a small portion of that signal.
With any radar absorption material (hereafter called RAM), the amount of absorption is proportional to the thickness of the coating. Thus, once the required degree of absorption is established, it is only necessary to ensure that the proper coating thickness has been applied to the aircraft. Therefore, any device that is to be used to test for the thickness of the coating should be small, compact, and hand holdable so that an individual can easily move it across the skin of the aircraft. Because most aircraft have complex contours, the testing device should also have a relatively small sensing head so as to be capable of following these contours. Since it is quite likely that the testing device will be used in the "field", it is also important that the device be portable and, wherever possible, "off-the-shelf" parts should be used to shorten repair times.
Measuring coating thickness is very important, especially when the coating is applied over a metal substrate, for most metals have little, if any, absorptive capability. Sometimes the coating is applied over substrates that are at least partially absorptive themselves. Thus, it is also important that the device be capable of measuring the total absorption level (coating and substrate) as well as the coating thickness alone.
One of the primary devices used to measure a non-conductive coating on a metal substrate is the well-known eddy current measuring device. The theory behind this device is well known and its details need not be discussed herein. In operation, the strength of the eddy current field stimulated in the metallic substrate decreases as the spacing between the probe and metal surface increases. Thus, a measurement of a non-conductive coating on a metal substrate can easily be obtained. But, such a device is limited to use with metal substrates and non-conductive coatings. Also, it would not be effective in measuring the aborption characteristics of a RAM coating applied over a substrate which also absorbs radar energy.
Various ultrasonic techniques are available to measure the thickness of coatings, but such devices require a coupling agent, usually water, which makes their use in the field, i.e., portability, limited. In addition, they can not measure the radar absorption characteristics of a combination coating and radar absorbing substrate.
Other apparatus of interest can be found in U.S. Pat. No. 3,988,582, "Blown Film Thickness Gauge" by R. W. Harman. Harman discloses an apparatus for measuring the thickness of plastic film as it is produced by a blown film-making machine. A radiation source and detector are placed on one side of the plastic film while a metal member is placed on the opposite side. The detector receives fluorescent X-rays from the metal member produced by a source of protons. The intensity of the scattered source of protons vary as a function of film thickness and with the intensity of the fluorescent X-rays. It is obvious that such an apparatus is unusable to measure the RAM coating thickness or the effectiveness of a RAM coating and substrate.
Another device of interest is disclosed in U.S. Pat. No. 4,042,723, "Method for Monitoring the Properties of Plastic Coatings on Optical Fibers" by H. M. Presby. The uniformity and concentricity of plastic coatings on an optical fiber are monitored by comparing the backscattered light patterns generated by two mutually perpendicular light beams incident upon the coated fiber. Unfortunately, this apparatus can only detect the thickness of optically clear material. It would have no application in determining the thickness of RAM coatings or the effectiveness of RAM coatings and substrate combinations.
Listed below are patents uncovered in a search of the prior art. They mainly deal with the measurement of the thickness of coatings by means of X-ray, gamma, or beta radiation, which generally depend upon monitoring backscatter to determine the thickness. None is felt to be relevant.
______________________________________ Pat. No. Title and Inventor ______________________________________ 3,076,894 Gamma Ray Thickness Gauges by J. L. Putman, et al. 3,115,577 Measuring Table for use in Coating Thickness Measuring by B. B. Joffe, et al. 3,497,691 Dual Mode Fluorescence and Backscatter Coating Thickness Measuring Gauge by Y. M. Chen 3,854,042 Device for Measuring the Thickness of Layers with a Radionuclide Irradiating the Layer by A. Ott 4,047,029 Self-Compensating X-ray or X-ray Thickness Gauge by J. J. Allport ______________________________________
Therefore, it is a primary object of the subject invention to provide a device to measure the thickness of a RAM coating.
It is a further object of the subject invention to provide a RAM coating thickness testing device that is portable and can be easily used to check the thickness of a coating on an aircraft surface.
A still further object of the subject invention is to provide a RAM coating thickness testing device that primarily makes use of standard parts.
Another object of the subject invention is to provide a RAM coating thickness testing device that can measure the effectiveness of both the coating and substrate upon which the coating is applied.